1. Field of the Invention
The present invention relates to a method of coating a cylindrical photoconductive element for use in an electrophotographic image forming apparatus and an apparatus therefore. More particularly, the present invention relates to a method capable of uniformly coating a conductive base with a liquid for forming a photoconductive layer and capable of coating a plurality of conductive bases with the liquid at the same time in a limited space, and an apparatus therefor.
2. Description of the Background Art
A photoconductive drum or similar photoconductive element for use in an electrophotographic image forming apparatus includes a cylindrical conductive base coated with a coating liquid that forms a photoconductive layer. For coating the conductive base with the coating liquid, use is generally made of a spray coater, a roll coater, a blade coater, a ring coater or similar coater or immersion coating. Among them, immersion coating holds the conductive base in a vertical position, dips the base in the coating liquid stored in a bath, and then lifts the base at a speed that sequentially varies to thereby form a photoconductive layer on the base.
More specifically, the above immersion coating generally includes a coating step, a peeling step, and a drying step. In the coating step, the conductive base is coated with the coating liquid that may additionally include an under layer forming liquid and a protection layer forming liquid. In the peeling step, needless portions of the photoconductive layer are peeled off the opposite end portions of the base. Subsequently, in the drying step, the photoconductive layer on the conductive base is dried either naturally or by heat, completing the photoconductive element.
To enhance productivity and reduce equipment cost, an immersion coating apparatus capable of saving space, and yet coating as great a number of conductive bases as possible at the same time, is required. Such an immersion coating apparatus has the following problem to be solved. The coating liquid contains a quick-drying solvent and therefore quickly dries and solidifies in a short period of time. However, during the interval between the lift of the conductive base away from the bath and drying to touch, the base is subjected to a light stream of air flowing therearound and to the vapor of the solvent produced from the photoconductive layer. Further, positioning a plurality of conductive bases in a limited space reduces a space available between nearby bases, so that each base is effected even by the flow of the vapor of the solvent produced from adjoining bases. In these conditions, the photoconductive layer or film formed on the individual base is irregular in thickness. An image forming apparatus using the resulting photoconductive drum brings about irregular density, background contamination and other defects in halftone images.
In light of the above, Japanese Patent No. 2,889,513 and Japanese Patent Laid-Open Publication No. 59-90662, for example, propose to arrange a windbreak on the top of a bath (scheme 1 hereinafter). Japanese Patent Laid-Open Publication Nos. 63-66560, for example, teaches a hood for enclosing a bath and cylindrical bodies (scheme 2 hereinafter). Japanese Patent Laid-Open Publication No. 7-144164, for example, proposes to lift bases together with a windbreak hood (scheme 3 hereinafter). Further, Japanese Patent Laid-Open Publication No. 63-7873, for example, proposes to mount a flexible hood on a base holder and immerse bases, which are enclosed by the hood, in a bath while sending air into the hood (scheme 4 hereinafter).
The scheme 1 is successful so long as the solvent of the coating liquid quickly dries to touch inside the windbreak. However, any delay in drying to touch disturbs the film thickness due to a light steam of air flowing above the windbreak. The scheme 2 has a problem that the hood must be large enough to enclose the entire bath, scaling up the equipment and increasing the cost. Another problem with the scheme 2 is that the vapor of a solvent is confined in the hood, causing much coating liquid to run down at the upper portion of each cylindrical body. The scheme 3 also needs a hood larger in size than the bath and higher than cylindrical bodies, scaling up equipment and increasing the cost. While the scheme 4 drives the vapor out of the hood via the bottom of the hood, it sends compressed air from a pump into the hood via the top of the hood. The compressed air therefore effects a film before the film dries to touch, resulting in irregular film thickness.